Question

How was a heavy isotope of nitrogen used to provide experimental evidence to semi-conservative mode of DNA-replication ?

Answer 1

Use of ¹⁵N helped to prove the semi-conservative replication of DNA.

Meselson and Stahl performed an experiment using E. coli to prove that DNA replication is semi-conservative. They grew E. coli in a medium containing ¹⁵N (the heavy isotope) and later transferred to ¹⁴N (the normal light isotope). Samples were taken from both of the media and their DNA was extracted. The heavy DNA molecule could be distinguished from the normal DNA by centrifugation in cesium chloride density gradient.

E. coli divides every 20 minutes. Therefore, the DNA extracted after 20 minutes had a hybrid density. DNA extracted after 40 minutes had equal amount of hybrid and light intensities. This implies that the newly synthesised DNA obtained one of its strands from the parent DNA molecule. Thus, Use of ¹⁵N proves that DNA replication is semi-conservative.